In my copending patent application Ser. No. 816,478 filed July 18, 1977 I describe the synthesis of 25-hydroxycholesterol from hyodeoxycholic acid. In this synthesis the steroid nucleus is protected from interaction with reagents subsequently utilized for the extension of the hyodeoxycholic acid side chain by one carbon atom.
Hyodeoxycholic acid has two hydroxyl groups in its steroid nucleus, a 3.alpha.- and a 6.alpha.-hydroxyl. At an early stage in this synthesis, the 6.alpha.-hydroxyl is eliminated and a 5,6-double bond introduced. The .alpha.-hydroxyl is epimerized to a 3.beta.-hydroxyl, and as disclosed in said application this 3.beta.-hydroxyl group is protected with a group that is resistant to the alkaline reducing agents which are involved in the side chain elongation procedure. Methoxyethoxymethyl (MEM) is such a group, and in said application I describe the formation of an MEM-ether which is the 3.beta.-(methoxyethoxymethoxy)-5-cholenic acid methyl ester. As disclosed, this MEM-ether may be formed by mixing 3.beta.-hydroxy-5-cholenic acid ester with MEM halide in the presence of diisopropyl ethylamine.
With the steroid nucleus so protected, the 3.beta.-(methoxyethoxymethoxy)-5-cholenic acid ester may be subjected to a series of steps in which it is treated with a reducing agent to reduce the 24-carboxylic ester groups to a 24-hydroxyl group suitably using as a reducing agent a complex of aluminum hydride with sodium, potassium or lithium, to obtain 3.beta.-(.beta.-methoxyethoxymethoxy)-24-hydroxy-4-cholene, and this may be mixed with p-toluenesulfonyl halide in pyridine solution and allowed to react to replace the hydroxyl at position 24 with OTs, and the resulting compound in turn may be reacted with a metallic cyanide such as sodium cyanide to replace the OTs group at position 24 with CN thus completing the extension of the carbon chain.
Then, as disclosed in my application Ser. No. 816,478, the steroid having the extended side chain may be treated with solid zinc bromide in methylene chloride solution to remove the MEM group and restore OH at the position 3 of the steroid nucleus.
One such treatment using zinc bromide for deprotection of certain MEM ethers is described by E. J. Corey, S. L. Graf and T. Ulrich in Tetrahedron Letters, 809 (1976). The resulting compound is 3.beta.-hydroxy-25-cyano-5-cholene.
In a further series of steps the cyano group of the 3.beta.-hydroxy-25-cyano-5-cholene is transformed into a carboxyl group by refluxing in ethanol solution with potassium hydroxide, and by acidification the free acid, 3.beta.-hydroxy-5-homocholenic acid, is obtained. This, in turn, may be refluxed in methanol solution with p-toluenesulfonic acid as a catalyst to obtain 3.beta.-hyroxy-5-homocholenic acid methyl ester. The resulting ester may be mixed with a methyl magnesium Grignard reagent in tetrahydrofuran and allowed to react to obtain 25-hydroxycholesterol which may be transformed to 25-hydroxy-7-dehydrocholesterol, and this, in turn, may be irradiated with ultraviolet light to obtain synthetic 25-hydroxycholecalciferol, a biologically active steroid, described by J. W. Blunt and H. F. DeLuca in Biochemistry 8, 671 (1969).
In repeated practice of the synthesis above outlined it was found that the yield of 3.beta.-hydroxy-5-homocholenic acid varied in different runs, which meant that for some reason the full yield of the desired steroid was not being consistently obtained. Accordingly, I set about to discover the reason for the failure to obtain uniformly high yields and to discover methods for correcting this difficulty.